1 Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of a magnification range of about 10 with a high relative aperture of about 1.4 in F-number, while still maintaining a good optical performance over the entire zooming range. Still more particularly it relates to compact zoom lenses of short total length suited to photographic cameras or video cameras.
2. Description of the Related Art
To the photographic cameras, video cameras, etc., there is a growing demand for zoom lenses of increased relative aperture and extended range while nevertheless maintaining high optical performance.
Of these, the zoom lens for video camera for home use is required to have as high a resolving power as, for example, 50 lines/mm in spatial frequency over the entire area of the picture frame, as the density of picture elements in the CCD or like image pickup element increases and the recording method such as the S-VHS is improved.
Besides this requirement, for the general-purpose video camera, because its image pickup element is relatively low in sensitivity, the zoom lens is required to have as large an aperture ratio as possible.
A zoom lens of which the F-number is about 1.4, and the range is about 10 is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Sho 54-17042 and 54-23556. This zoom lens comprises, from front to rear, a first lens unit of positive power for focusing, a second lens unit of negative power for variation of the image magnification, a third lens unit for compensating for the image shift resulting from the variation of the image magnification, a fourth lens unit for making the light bundle from the third lens unit an afocal light bundle, and a fifth lens unit for image formation. Hence, the above-cited documents propose the so-called 5-unit zoom lens.
With the use of the 5-unit type in the zoom lens design, to achieve great increases in the aperture ratio and the zoom ratio at once, it is effective in the general case to weaken the refractive power of each lens unit. Weakening of the refractive power, however, causes the total length of the lens system and the lens diameter to increase, which is scarcely favor to the video camera for home use, since its requirement for compact form is rigorous.
Also, if the F-number at full open aperture is made larger, it becomes easier to reduce the bulk of the lens diameter. But, for the video camera employing the low-sensitive image pickup element, the larger values of the F-number are of little use.
In general, to achieve a good compromise between the requirements of decreasing the F-number and of reducing the bulk and size of entirety of the lens system of increased zoom ratio, it becomes of great importance that the various design parameters for all the lens units constituting the lens system are set at appropriate values.
If merely relied on the method of strengthening the refractive power of each individual lens unit, the compact form and the increase of the aperture ratio of the lens system would result in production of large spherical aberration in the paraxial region of the picture format and large higher order aberrations such as coma and sagittal halo in the marginal zone, which can hardly be corrected for high grade imaging performance.
For example, the configuration of the lens system to the compact form by strengthening the refractive power of the front or first lens unit leads to the necessity of increasing the overall image magnifying power of the variator through the image forming unit. As a result, the first lens unit produces many aberrations which are difficult to correct by the suitable design of the following lens units. The manufacturing tolerances also become severer.
Also, the zoom section of the second and third lens units, when both of their refractive powers are strengthened to increase the magnification range, gives rise to a large range of variation with zooming of all aberrations, particularly meridional curvature of field and lateral chromatic aberration. And the difficulty of maintaining good correction of these aberrations stable throughout is increased.
Meanwhile, zoom lenses suited to video cameras are known in U.S. Pat. Nos. 4,518,228, 4,525,036, 4,618,219, 4,621,905, 4,653,874 and 4,659,187. In addition, there are Japanese Laid-Open Patent Applications Nos. Sho 59-222807, 60-260912 and 61-20291 and U.S. patent application Ser. No. 190,472 filed May 5, 1988.